Current SMB users may find themselves in a situation where they can’t run BGP. Perhaps their upstream ISP blocks the ability to establish a connection. In many cases, business class service is required with additional fees necessary to multihome. In order to take full advantage of independent links to different ISPs, two (or more) NAT configurations are required to send and receive packets correctly across the balanced connections. While technically feasible, it’s a mess to troubleshoot. It also doesn’t scale when multiple egress connections are configured. And more often that not, the configuration to make everything work correctly exists on a single router in the network, eliminating the advantages of multihoming.

LISP seeks to solve this by using a mapping database to send packets to the correct Ingress Tunnel Router (ITR) without the need for BGP. The diagram of a LISP packet looks a lot like an overlay. That’s because it is in many ways. The LISP packets are tunneled from an Egress Tunnel Router (ETR) to a LISP speaking decapsulation point. Depending on the deployment policies of LISP for a given ISP, it could be the next hop router on a connection. It could also be a router several hops upstream. LISP is capable of operating over non-LISP speaking connections, but it does eventually need decapsulation.

Where’s the Achille’s Heel in this design? LISP may solve the issue without BGP, but it does introduce the need for the LISP session to terminate on a single device (or perhaps a group of devices). This creates issues in the event the link goes down and the backup link needs to be brought online. That tunnel state won’t be preserved across the failover. It’s also a gamble to assume your ISP will support LISP. Many large ISPs should give you options to terminate LISP connections. But what about the smaller ISP that services many SMB companies? Does the local telephone company have the technical ability to configure a LISP connection? Let along making it redundant and highly available?

Right Tool For The Job

I think back to a lesson my father taught me about tools. He told me, “Son, you can use a screwdriver as a chisel if you try hard enough. But you’re better off spending the money to buy a chisel.” The argument against using BGP to multihome ISP connections has always come down to cost. I’ve gotten into heated discussions with people that always come back to the expense of upgrading to a business-class connection to run BGP or ensure availability. NAT may allow you to multihome across two residential cable modems, but why do you need 99.999% uptime across those two if you’re not willing to pay for it?

LISP solves one issue only to introduce more. I see LISP being misused the same way NAT has been. LISP was proposed by David Meyer to solve the exploding IPv4 routing table and the specter of an out-of-control IPv6 routing table. While multihoming is certainly another function that it can serve, I don’t think that was Meyer’s original idea. BGP might not be perfect, but it’s what we’ve got. We’ve been using it for a while and it seems to get the job done. LISP isn’t going to replace BGP by a long shot. All you have to do it look at LISP ALternate Topology (LISP-ALT), which was the first iteration of the mapping database before the current LISP-TREE. Guess what LISP-ALT used for mapping? That’s right, BGP.

Tom’s Take

LISP multihoming for IPv4 or IPv6 in SMEs isn’t going to fix the problem we have today with trying to create redundancy from consumer-grade connections. It is another overlay that will create some complexity and eventually not be adopted because there are still enough people out there that are willing to forgo an interesting idea simply because it came from Cisco. IPv6 multihoming can be fixed at the protocol level. Tuning router advertisements or configuring routes at the edge with BGP will get the job done, even if it isn’t as elegant as LISP. Using the right tool for the right job is the way to make multihoming happen.

One thought on “Is LISP The Answer to Multihoming?”

Good points, Tom. Although I’ve heard the arguments over and over, I’m also wary of a routing system that relies on a distributed database (a la DNS) and how that scales to maintain the performance needed for routing lookups.

The place I am interested in LISP maybe helping is with a number of my customers who feel they need ISP redundancy, but really can’t justify a /24 of IP space which is the smallest prefix that can generally be advertised on the Internet. Some customers only have 20-30 IPs in use with slow growth. It’s a real stretch to justify the need for a /24 to ARIN (25% utilization in the first 30 days and 50% after 1 year), or even a carrier (the ISPs are getting even more stingy than ARIN at this point, in my experience). LISP has potential to allow a customer to effectively multihome a /26 or even smaller prefix.

I know my customers will need to see LISP with wider deployment and support before adopting it as a means of providing missions critical connectivity to their office or data center, though.